U.S. patent number 5,681,405 [Application Number 08/401,418] was granted by the patent office on 1997-10-28 for method for making an improved aluminum alloy sheet product.
This patent grant is currently assigned to Golden Aluminum Company. Invention is credited to William Newton, David A. Tomes.
United States Patent |
5,681,405 |
Newton , et al. |
October 28, 1997 |
Method for making an improved aluminum alloy sheet product
Abstract
An aluminum alloy sheet and a method for producing an aluminum
alloy sheet. The aluminum alloy sheet is useful for forming into
drawn and ironed container bodies. The sheet preferably has an
after-bake yield strength of at least about 37 ksi and an
elongation of at least about 2 percent. Preferably the sheet also
has earing of less than about 2 percent.
Inventors: |
Newton; William (San Antonio,
TX), Tomes; David A. (San Antonio, TX) |
Assignee: |
Golden Aluminum Company
(Golden, CO)
|
Family
ID: |
23587670 |
Appl.
No.: |
08/401,418 |
Filed: |
March 9, 1995 |
Current U.S.
Class: |
148/551; 148/439;
148/552; 148/692; 420/533; 420/534 |
Current CPC
Class: |
C22C
21/00 (20130101); C22C 21/06 (20130101); C22F
1/04 (20130101); C22F 1/047 (20130101) |
Current International
Class: |
C22C
21/00 (20060101); C22C 21/06 (20060101); C22F
1/047 (20060101); C22F 1/04 (20060101); C22F
001/04 () |
Field of
Search: |
;148/551,552,688,692,696,439 ;420/533,534,537,538,542,546,547 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Don McAuliffe, "Production of Continuous Cast Can Body Stock",
Paper presented at AIME Meeting, Feb. 27, 1989, 7 pages..
|
Primary Examiner: Wyszomierski; George
Attorney, Agent or Firm: Sheridan Ross P.C.
Claims
What is claimed is:
1. A method for fabricating an aluminum sheet product, comprising
the steps of:
(a) forming an aluminum alloy melt comprising;
(i) from about 0.7 to about 1.3 weight percent manganese,
(ii) from about 1.0 to about 1.5 weight percent magnesium,
(iii) from about 0.3 to about 0.6 weight percent copper,
(iv) up to about 0.5 weight percent silicon, and
(v) from about 0.3 to about 0.7 weight percent iron, the balance
being aluminum and incidental additional materials and
impurities;
(b) continuously casting said alloy melt to form a cast strip;
(c) hot rolling said cast strip to reduce the thickness of said
cast strip and form a hot rolled strip;
(d) cold rolling said hot rolled strip to form a cold rolled strip
wherein the thickness of said hot rolled strip is reduced by from
about 35 percent to about 60 percent per pass;
(e) annealing said cold rolled strip to form an intermediate cold
mill annealed strip; and
(f) further cold rolling said intermediate cold mill annealed strip
to reduce the thickness of the strip and form aluminum alloy strip
stock;
wherein said aluminum alloy strip stock has an after-bake yield
strength of at least about 37 ksi and an earing of less than about
2 percent.
2. A method as recited in claim 1, wherein said aluminum alloy melt
comprises from about 0.35 to about 0.5 weight percent copper.
3. A method as recited in claim 1, wherein said hot rolling step
reduces the gauge of said cast strip by at least about 70
percent.
4. A method as recited in claim 1, wherein said method comprises
the step of either:
(i) annealing said hot rolled strip for at least about 0.5 hour at
a temperature of from about 700.degree. F. to about 900.degree. F.
to form a hot mill annealed strip; or
(ii) cooling said hot rolled strip;
immediately after said hot rolling step.
5. A method as recited in claim 1, further comprising the step of
annealing said hot rolled strip immediately after said hot rolling
step for at least about 0.5 hour at a temperature of from about
700.degree. F. to about 900.degree. F.
6. A method as recited in claim 5, wherein said step of annealing
said hot rolled strip comprises heating said hot rolled strip at a
temperature of from about 800.degree. F. to about 850.degree.
F.
7. A method as recited in claim 6, wherein said step of annealing
said cold rolled strip comprises annealing said cold rolled strip
for about 3 hours.
8. A method as recited in claim 5, wherein the cooling of said
strip from said hot mill annealing step is for at least about 0.5
hour.
9. A method as recited in claim 1, wherein said step of annealing
said hot rolled strip comprises annealing said hot rolled strip for
from about 1 to about 5 hours.
10. A method as recited in claim 1, wherein said step of annealing
said cold rolled strip comprises annealing said cold rolled strip
at a temperature of from about 600.degree. F. to about 900.degree.
F. in a batch anneal oven.
11. A method as recited in claim 1, wherein said aluminum alloy
strip stock has an elongation of at least about 2 percent.
12. A method as recited in claim 1, wherein said step of further
cold rolling said cold mill annealed strip comprises cold rolling
said cold mill annealed strip to reduce the thickness of said cold
mill annealed strip by from about 45 percent to about 80
percent.
13. A method as recited in claim 1, wherein said step of hot
rolling said cast strip occurs sequentially after said step of
continuously casting without any intermediate heat treatment
step.
14. A method as recited in claim 1, wherein said aluminum alloy
melt comprises at least about 75 weight percent scrap.
15. A method as recited in claim 1, wherein said aluminum alloy
melt comprises at least about 95 weight percent scrap.
16. A method as recited in claim 1, wherein said iron level is
selected such that the resultant microstructure, in said strip
stock is a fine grain microstructure.
17. A method as recited in claim 1, further comprising the step of
forming said aluminum strip stock into drawn and ironed
containers.
18. A method as recited in claim 1, wherein the annealing of said
cold rolled strip is at a temperature of from about 800.degree. F.
to about 1050.degree. F. in a continuous anneal step.
19. An aluminum sheet product produced by the method of claim
1.
20. A method for fabricating an aluminum alloy strip stock,
comprising the steps of:
(a) forming an aluminum alloy melt derived from at least about 75
weight percent scrap, comprising;
(i) from about 0.7 to about 1.3 weight percent manganese;
(ii) from about 1.0 to about 1.5 weight percent magnesium;
(iii) from about 0.35 to about 0.5 weight percent copper;
(iv) up to about 0.5 weight percent silicon; and
(v) from about 0.4 to about 0.65 weight percent iron, the balance
being aluminum and incidental additional materials and
impurities;
(b) continuously casting said alloy melt to form a cast strip;
(c) hot rolling said cast strip to reduce the thickness of said
cast strip by at least about 70 percent to form a hot rolled
strip;
(d) annealing said hot rolled strip for at least about 0.5 hour at
a temperature of from about 700.degree. F. to about 900.degree. F.
to form a hot mill annealed strip;
(e) cooling said hot mill annealed strip for at least about 0.5
hour;
(f) cold rolling said hot mill annealed strip to form a cold rolled
strip wherein the thickness of said hot mill annealed strip is
reduced by from about 35% to about 60% per pass;
(g) annealing said cold rolled strip to form a cold mill annealed
strip by either:
(i) batch annealing at a temperature of from about 650.degree. F.
to about 750.degree. F.; or;
(ii) continuous annealing at a temperature of from about
800.degree. F. to about 1050.degree. F.; and
(h) further cold rolling said cold mill annealed strip to reduce
the thickness of the strip and form aluminum alloy strip stock;
wherein said aluminum alloy strip stock has an after-bake yield
strength of at least about 37 ksi and an eating of less than about
2 percent.
21. An aluminum alloy strip stock produced by the process of claim
20.
22. Aluminum alloy strip stock produced by continuous casting,
comprising:
(a) from about 0.7 to about 1.3 weight percent manganese;
(b) from about 1.0 to about 1.5 weight percent magnesium;
(c) from about 0.38 to about 0.45 weight percent copper;
(d) from about 0.50 to about 0.60 weight percent iron;
(e) up to about 0.5 weight percent silicon, the balance being
aluminum and incidental additional materials and impurities;
wherein said strip stock has an after-bake yield strength of at
least about 37 ksi and earing of less than about 2 percent.
23. The aluminum alloy strip stock as claimed in claim 22,
comprising from about 0.75 to about 1.2 weight percent
manganese.
24. The aluminum alloy strip stock of claim 22, comprising from
about 0.80 to about 1.1 weight percent manganese.
25. The aluminum alloy strip stock of claim 22, comprising from
about 1.15 to about 1.45 weight percent magnesium.
26. The aluminum alloy strip stock of claim 22, comprising from
about 1.2 to about 1.4 weight percent magnesium.
27. The aluminum alloy strip stock of claim 22, comprising from
about 0.13 to about 0.25 weight percent silicon.
28. The aluminum alloy strip stock of claim 22, wherein said strip
stock has an after-bake yield strength of at least 38 ksi.
29. The aluminum alloy strip stock of claim 22, wherein said strip
stock has an after-bake yield strength of at least 40 ksi.
30. The aluminum alloy strip stock of claim 22, wherein said strip
stock has an after-bake ultimate tensile strength of at least 40
ksi.
31. The aluminum alloy strip stock of claim 22, wherein said strip
stock has an after-bake ultimate tensile strength of at least 41.5
ksi.
32. The aluminum alloy strip stock of claim 22, wherein said strip
stock has an after-bake ultimate tensile strength of at least 43
ksi.
33. The aluminum alloy strip stock of claim 22, wherein said strip
stock has earing of less than 1.8 percent.
34. The aluminum alloy strip stock of claim 22, wherein said strip
stock has an elongation of greater than 2.0 percent.
35. The aluminum alloy strip stock of claim 22, wherein said strip
stock has an elongation of greater than 3.0 percent.
36. The aluminum alloy strip stock of claim 22, wherein said strip
stock has an elongation of greater than 4.0 percent.
37. The aluminum alloy strip stock of claim 22, wherein said strip
stock is capable of being made into a drawn and ironed container
having an average dome thickness of from about 0.0096 inches to
about 0.015 inches and a minimum dome reversal strength of about 90
psi.
38. An aluminum alloy sheet produced by a method comprising the
steps of:
(a) forming an aluminum alloy melt comprising;
(i) from about 0.7 to about 1.3 weight percent manganese,
(ii) from about 1.0 to about 1.5 weight percent magnesium,
(iii) from about 0.3 to about 0.6 weight percent copper,
(iv) up to about 0.5 weight percent silicon, and
(v) from about 0.3 to about 0.7 weight percent iron, the balance
being aluminum and incidental additional materials and
impurities;
(b) continuously casting said alloy melt to form a cast strip;
(c) hot rolling said cast strip to reduce the thickness of said
cast strip and form a hot rolled strip;
(d) annealing said hot rolled strip for at least about 0.5 hour at
a temperature of from about 700.degree. F. to about 900.degree. F.
to form a hot mill annealed strip;
(e) cold rolling said hot mill annealed strip to form a cold rolled
strip wherein the thickness of said hot mill annealed strip is
reduced by from about 35 percent to about 60 percent per pass;
(f) annealing said cold rolled strip by either:
(i) batch annealing at a temperature of from about 600.degree. F.
to about 900.degree. F. to form a cold mill annealed strip; or
(ii) continuous annealing at a temperature from about 800.degree.
F. to about 1050.degree. F. to form a cold mill annealed strip;
and
(g) further cold rolling said cold mill annealed strip to reduce
the thickness of the strip and form aluminum alloy strip stock;
wherein said aluminum alloy strip stock has an after-bake yield
strength of at least about 37 ksi and an earing of less than about
2 percent.
Description
FIELD OF THE INVENTION
The present invention relates generally to aluminum alloy sheet and
methods for making aluminum alloy sheet. Specifically, the present
invention relates to aluminum alloy sheet and methods for making
aluminum alloy sheet wherein the sheet is particularly useful for
forming into drawn and ironed container bodies.
BACKGROUND OF THE INVENTION
Aluminum beverage containers are generally made in two pieces, one
piece forming the container sidewalls and bottom (referred to
herein as a "container body") and a second piece forming the
container top. Container bodies are formed by methods well known in
the art. Generally, the container body is fabricated by forming a
cup from a circular blank of aluminum sheet and then extending and
thinning the sidewalls by passing the cup through a series of dies
having progressively smaller bore size. This process is referred to
as "drawing and ironing" the container body.
A common aluminum alloy used to produce container bodies is AA3004,
an alloy registered with the Aluminum Association. The physical
characteristics of AA 3004 are appropriate for drawing and ironing
container bodies due primarily to the relatively low magnesium (Mg)
and manganese (Mn) content of the alloy. A desirable characteristic
of AA3004 is that the amount of work hardening imparted to the
aluminum sheet during the can making process is relatively
minor.
Aluminum alloy sheet is most commonly produced by an ingot casting
process. In this process, the aluminum alloy material is initially
cast into an ingot, for example having a thickness of from about 20
to 30 inches. The ingot is then homogenized by heating to an
elevated temperature, which is typically 1075.degree. F. to
1150.degree. F., for an extended period of time, such as from about
6 to 24 hours. The homogenized ingot is then hot rolled in a series
of passes to reduce the thickness of the ingot. The hot rolled
sheet is then cold rolled to the desired final gauge.
Despite the widespread use of ingot casting, there are numerous
advantages to producing aluminum alloy sheet by continuously
casting molten metal. In a continuous casting process, molten metal
is continuously cast directly into a relatively long thin slab and
the cast slab is then hot rolled and cold rolled to produce a
finished product. However, not all alloys can be readily cast using
a continuous casting process into aluminum sheet that is suitable
for forming operations, such as for making drawn and ironed
container bodies.
Attempts have been made to continuously cast AA 3004 alloy. For
example, in a paper entitled "Production of Continuous Cast Can
Body Stock," which was presented by McAuliffe, an employee of the
assignee of the present application, on Feb. 27, 1989, at the AIME
meeting in Las Vegas, it is disclosed that limited testing was
conducted with two manufacturers of 12 ounce, 90 pound cans (i.e.,
a minimum buckle strength of 90 p.s.i.). One test produced 3004 can
stock. The paper discloses that "[b]oth tests, in the 2-3% earing
range, verified that the surface and internal quality and structure
were sufficient to produce cans of acceptable quality." However, it
has been found that the continuously cast AA3004 alloy is
unsuitable for typical high carbonation beverages, such as soda,
because it has insufficient buckle strength when employed using
current typical stock gauges (e.g., from about 0.0112" to 0.0118")
as opposed to stock gauges used at the time of the McAuliffe
article (e.g., from about 0.0124" to 0.0128"). This is due to the
poor after-bake characteristics of continuously cast AA 3004 alloy
that is produced having suitable eating levels. This is discussed
in more detail hereinafter in connection with examples of the
physical characteristics of continuously cast AA 3004 alloy.
U.S. Pat. No. 4,238,248 by Gyongos et al. discloses casting an AA
3004 type alloy in a block casting apparatus. The alloy had a
magnesium content from 0.8 to 1.3 percent and a manganese content
from 1.0 to 1.5 percent, with up to 0.25 percent copper. As used
throughout the present specification, all percentages refer to
weight percent unless otherwise indicated. However, there is no
disclosure of processing the cast strip into sheet suitable for
container bodies.
U.S. Pat. No. 4,235,646 by Neufeld et al. describes the continuous
casting of an AA5017 aluminum alloy that is useful for beverage
container bodies and container ends. The alloy includes 0.4 to 1.0
percent manganese, 1.3 to 2.5 percent magnesium and 0.05 to 0.4
percent copper. However, it is also disclosed that "copper and iron
are included in the present composition due to their inevitable
presence in consumer scrap. The presence of copper between 0.05 and
0.2 percent also enhances the low earing properties and adds to the
strength of the present alloy." In Examples 1-3, the copper content
of the alloys was 0.04 percent and 0.09 percent. In addition, the
process includes a flash anneal step. In one example, the sheet
stock disclosed by Neufeld et al. had a yield strength after cold
rolling of 278 MPa (40.3 ksi) and an earing percentage of 1.2
percent.
U.S. Pat. No. 4,976,790 by McAuliffe et al. discloses a process for
casting aluminum alloys using a block-type strip caster. The
process includes the steps of continuously casting an aluminum
alloy strip and thereafter introducing the strip into a hot mill at
a temperature of from about 880.degree. F. to 1000.degree. F.
(471.degree. C.-538.degree. C.). The strip is hot rolled to reduce
the thickness by at least 70 percent and the strip exits the hot
roll at a temperature of no greater than 650.degree. F.
(343.degree. C.). The strip is then coiled to anneal at 600.degree.
F. to 800.degree. F. (316.degree. C.-427.degree. C.) and is then
cold rolled, annealed and subjected to further cold rolling to
optimize the balance between the 45.degree. earing and the yield
strength. The preferred annealing temperature after cold rolling is
695.degree. F. to 705.degree. F. (368.degree. C.-374.degree.
C.).
U.S. Pat. No. 4,517,034 by Merchant et al. describes a method for
continuously casting a modified AA 3004 alloy composition which
includes 0.1 to 0.4 percent chromium. The sheet stock has an eating
percentage of 3.12 percent or higher.
U.S. Pat. No. 4,526,625 by Merchant et al. also describes a method
for continuously casting an AA 3004 alloy composition which is
alleged to be suitable for drawn and ironed container bodies. The
process includes the steps of continuously casting an alloy,
homogenizing the cast alloy sheet at 950.degree. F.-1150.degree. F.
(510.degree. C.-621.degree. C.), cold rolling the sheet, and
annealing the sheet at 350.degree. F.-550.degree. F. (177.degree.
C.-288.degree. C.) for a time of about 2-6 hours. The sheet is then
cold rolled and reheated to recrystallize the grain structure at
600.degree. F.-900.degree. F. (316.degree. C.-482.degree. C.) for
about 1-4 hours. The sheet is then cold rolled to final gauge. The
reported earing for the sheet is about 3 percent or higher.
U.S. Pat. No. 5,192,378 by Doherty et al. discloses a process for
making an aluminum alloy sheet useful for forming into container
bodies. The aluminum alloy includes 1.1-1.7 percent magnesium,
0.5-1.2 percent manganese and 0.3-0.6 percent copper. The cast
ingot is homogenized at 900.degree. F.-1080.degree. F. for about 4
hours, hot rolled, annealed at 500.degree. F.-700.degree. F., cold
rolled and then annealed at 750.degree.-1050.degree. F. The body
stock can have a yield strength of 40-52 ksi after the final cold
rolling.
U.S. Pat. No. 4,111,721 by Hitchler et al. discloses a process for
continuously casting AA 3004 type alloys. The cast sheet is held at
a temperature of at least about 900.degree. F. (482.degree. C.) for
from about 4 to 24 hours prior to final cold reduction.
European Patent Application No. 93304426.5 discloses a method and
apparatus for continuously casting aluminum alloy sheet. It is
disclosed that an aluminum alloy having 0.93 percent manganese,
1.09 percent magnesium and 0.42 percent copper and 0.48 percent
iron was cast into a strip. The composition was hot rolled in two
passes and then solution heat treated continuously for 3 seconds at
1000.degree. F. (538.degree. C.), quenched and cold rolled to final
gauge. Can bodies made from the sheet had an caring of 2.8 percent,
a tensile yield strength of 43.6 ksi (301 MPa). An important aspect
of the invention disclosed in European Patent Application No.
93304426.5 is that the continuously cast strip be subjected to
solution heat treating immediately after hot rolling without
intermediate cooling, followed by a rapid quench. In fact, it is
illustrated in Example 4 that strength is lost when the solution
heat treatment and quenching steps of the invention are replaced
with a conventional batch coil annealing cycle and cold working is
limited to about 50 percent to maintain required eating, as is
typical in continuous cast processes. Solution heat treating is
disadvantageous because of the high capital cost of the necessary
equipment and the increased energy requirements.
There remains a need for a process which produces an aluminum alloy
sheet having sufficient strength and formability characteristics to
be easily made into drawn and ironed beverage containers. The sheet
stock should have good strength and elongation, and the resulting
container bodies should have low caring.
It would be desirable to have a continuous aluminum casting process
in which there is no need for a heat soak homogenization step. It
would be advantageous to have a continuously cast process in which
it is unnecessary to continuously anneal and solution heat treat
the cast strip immediately following hot rolling (e.g., without
intermediate cooling) followed by immediate quenching. It would be
advantageous to have an aluminum alloy suitable for continuous
casting in which the grain size is sufficient to provide for
enhanced formability. It would be desirable to have an aluminum
alloy suitable for continuous casting in which the magnesium level
is kept low in order to achieve comparable brightness when compared
to commercially available continuous cast can stock. It would be
desirable to have an aluminum alloy suitable for continuous casting
which can be formed into containers having suitable formability and
having low caring and suitable strength.
SUMMARY OF THE INVENTION
In accordance with the present invention, a method is provided for
fabricating an aluminum sheet product. The method includes the
following steps. An aluminum alloy melt is formed which includes
from about 0.7 to about 1.3 weight percent manganese, from about
1.0 to about 1.5 weight percent magnesium, from about 0.3 to about
0.6 weight percent copper, up to about 0.5 weight percent silicon,
and from about 0.3 to about 0.7 weight percent iron, the balance
being aluminum and incidental additional materials and impurities.
In a preferred embodiment, the aluminum alloy melt includes from
about 1.15 to about 1.45 weight percent magnesium and more
preferably from about 1.2 to about 1.4 weight percent magnesium,
from about 0.75 to about 1.2 weight percent manganese and more
preferably from about 0.8 to about 1.1 weight percent manganese,
from about 0.35 to about 0.5 weight percent copper and more
preferably from about 0.38 to about 0.45 weight percent copper,
from about 0.4 to about 0.65 weight percent iron and more
preferably from about 0.50 to about 0.60 weight percent iron, and
from about 0.13 to about 0.25 weight percent silicon, with the
balance being aluminum and incidental additional materials and
impurities. The alloy melt is continuously cast to form a cast
strip and the cast strip is hot rolled to reduce the thickness and
form a hot rolled strip. The hot rolled strip can be subsequently
cold rolled without any intervening hot mill anneal step or can be
annealed after hot rolling for at least about 0.5 hours at a
temperature from about 700.degree. F. to about 900.degree. F. to
form a hot mill annealed strip. The hot rolled strip or hot mill
annealed strip is cold rolled to form a cold rolled strip wherein
the thickness of the strip is reduced to the desired intermediate
anneal gauge, preferably by about 35% to about 60% per pass. The
cold rolled strip is annealed to form an intermediate cold mill
annealed strip. The intermediate cold mill annealed strip is
subjected to further cold rolling to reduce the thickness of the
strip and form aluminum alloy strip stock.
In accordance with the present invention, aluminum alloy strip
stock is provided comprising from about 0.7 to about 1.3 weight
percent manganese, from about 1.0 to about 1.5 weight percent
magnesium, from about 0.38 to about 0.45 weight percent copper,
from about 0.50 to about 0.60 weight percent iron and up to about
0.5 weight silicon, with the balance being aluminum and incidental
additional materials and impurities. The aluminum alloy strip stock
is preferably made by continuous casting. Preferably, the strip
stock has a final gauge after-bake yield strength of at least about
37 ksi, more preferably at least about 38 ksi and more preferably
at least about 40 ksi. The strip stock preferably has an eating of
less than 2 percent and more preferably less than 1.8 percent.
In accordance with the present invention, a continuous process for
producing aluminum sheet is provided. In accordance with the
process, relatively high reductions in gauge can be achieved in
both the hot mill and cold mill. Additionally, due to the fact that
greater hot mill and cold mill reductions are possible, the number
of hot roll and cold roll passes can be reduced as compared to
commercially available continuously cast can body stock. A
relatively high proportion of cold work is needed to produce can
body stock having acceptable physical properties according to the
sheet production process of the present invention, as compared to
commercially available continuously cast can body stock. Thus, a
reduced amount of work hardening is imparted to the sheet when it
is manufactured into items such as drawn and ironed containers,
when compared to commercially available continuously cast can body
stock.
In accordance with the present invention, the need for a high
temperature soak (i.e., homogenization) can be avoided. When the
high temperature homogenization step is performed when the metal is
coiled, it can result in pressure welding such that it is
impossible to unroll the coil. Also, the need for solution heat
treatment after the hot mill (e.g., as disclosed in European Patent
Application No. 93304426.5) can be avoided. By avoiding solution
heat treatment, the continuous casting process is more economical
and results in fewer process control problems.
In accordance with the present process, high amounts of recycled
aluminum can be advantageously employed. For example, 75 percent
and preferably up to 95 percent or more of used beverage containers
(UBC) can be employed to produce the continuous cast sheet of the
present invention. The use of increased amounts of UBC
significantly reduces the cost associated with producing the
aluminum sheet.
In accordance with the present invention, a continuous cast alloy
is provided which includes relatively high levels of copper (e.g.,
0.3 to 0.6 percent). It has surprisingly been found that the copper
can be increased to these levels without negatively affecting the
eating. If copper is increased in ingot cast processes, the
resulting alloy can be too strong for can-making applications. In
addition, in accordance with the present invention, relatively low
levels of magnesium are used (e.g., 1.0 to 1.5 percent), leading to
better can surface finish than commercially available continuously
cast can body stock. For example, when drawn and ironed cans
manufactured from aluminum sheet according to the present invention
are subjected to industrial washing, less surface etching takes
place and, therefore, a brighter can results. Also, the relatively
low magnesium content decreases the work hardening rate. Also in
accordance with the present invention, a relatively high iron
content compared to commercially available continuous cast can body
stock is employed to increase formability. It is believed that
formability is increased because the increased iron changes the
microstructure resulting in a finer grain material, when compared
to a low iron content continuously cast material. The tolerance of
these high iron levels also increases the amount of UBC that can be
utilized, since iron is a common contaminant in consumer scrap
.
BRIEF DESCRIPTION OF THE DRAWING
The FIGURE is a block diagram illustrating one embodiment of the
process of the present invention.
DETAILED DESCRIPTION
In accordance with the present invention, aluminum sheet having
good strength and forming properties is provided. In addition, a
process for producing aluminum sheet is also provided. The
resulting aluminum sheet is particularly suitable for the
fabrication of drawn and ironed articles, such as containers. The
resulting sheet has reduced earing and improved strength in thinner
gauges than comparable sheet fabricated according to the prior
art.
The preferred aluminum alloy composition according to the present
invention includes the following constituents: (1) manganese,
preferably with a minimum of at least about 0.7 percent manganese
and more preferably with a minimum of at least about 0.75 percent
manganese and more preferably with a minimum of at least about 0.8
percent manganese, and preferably with a maximum of at most about
1.3 percent manganese and more preferably with a maximum of at most
about 1.2 percent manganese and more preferably with a maximum of
at most about 1.1 percent manganese; (2) magnesium, preferably with
a minimum of at least about 1.0 percent magnesium and more
preferably with a minimum of at least about 1.15 percent magnesium
and more preferably with a minimum of at least about 1.2 percent
magnesium, and preferably with a maximum of at most about 1.5
percent magnesium and more preferably with a maximum of at most
about 1.45 percent magnesium and more preferably with a maximum of
at most about 1.4 percent magnesium; (3) copper, preferably with a
minimum of at least about 0.3 percent copper and more preferably
with a minimum of at least about 0.35 percent copper and more
preferably with a minimum of at least about 0.38 percent copper,
and preferably with a maximum of at most about 0.6 percent copper
and more preferably with a maximum of at most about 0.5 percent
copper and more preferably with a maximum of at most about 0.45
percent copper; (4) iron, preferably with a minimum of at least
about 0.3 percent iron and more preferably with a minimum of at
least about 0.4 percent iron and more preferably with a minimum of
at least about 0.50 percent iron, and preferably with a maximum of
at most about 0.7 percent iron and more preferably with a maximum
of at most about 0.65 percent iron and more preferably with a
maximum of at most about 0.60 percent iron; (5) silicon, preferably
with a minimum of 0 percent silicon and more preferably with a
minimum of at least about 0.13 percent silicon, and preferably with
a maximum of at most about 0.5 percent silicon and more preferably
with a maximum of at most about 0.25 percent silicon. The balance
of the alloy composition consists essentially of aluminum and
incidental additional materials and impurities. The incidental
additional materials and impurities are preferably limited to about
0.05 weight percent each, and the sum total of all incidental
additional materials and impurities preferably does not exceed
about 0.15 percent.
While not wishing to be bound by any theory, it is believed that
the copper content of the alloy composition according to the
present invention, particularly in combination with the process
steps discussed below, contributes to the increased strength of the
aluminum alloy sheet stock while maintaining acceptable elongation
and earing characteristics. Additionally, it is believed that the
relatively low level of magnesium results in a brighter finish in
containers manufactured from the alloy of the present invention,
due to a decrease in surface etching, when compared to currently
commercially available continuously cast stock. Furthermore, it is
believed that the relatively high level of iron leads to increased
formability because the iron changes the microstructure resulting
in a finer grain material when compared to continuous cast
materials cast with similar levels of manganese, copper and
magnesium and, having lower levels of iron.
According to a preferred embodiment of the present invention, a
continuous casting process is used to form an aluminum alloy melt
into an aluminum alloy sheet product. The continuous casting
process can employ a variety of continuous casters, such as a belt
caster or a roll caster. Preferably, the continuous casting process
includes the use of a block caster for casting the aluminum alloy
melt into a sheet. The block caster is preferably of the type
disclosed in U.S. Pat. Nos. 3,709,281; 3,744,545; 3,747,666;
3,759,313 and 3,774,670 all of which are incorporated herein by
reference in their entirety.
According to this embodiment of the present invention, a melt of
the aluminum alloy composition described above is formed. The alloy
composition according to the present invention can be formed in
part from scrap material such as plant scrap, can scrap and
consumer scrap. Plant scrap can include ingot scalpings, rolled
strip slicings and other alloy trim produced in the mill operation.
Can scrap can include scrap produced as a result of earing and
galling during can manufacture. Consumer scrap can include
containers recycled by users of beverage containers. It is
preferred to maximize the amount of scrap used to form the alloy
melt and preferably the alloy composition according to the present
invention is formed with at least about 75 percent and preferably
at least about 95 percent total scrap.
In order to come within the preferred elemental ranges of the
present alloy, it is necessary to adjust the melt. This may be
carried out by adding elemental metal, such as magnesium or
manganese, or by adding unalloyed aluminum to the melt composition
to dilute excess alloying elements.
The metal is charged into a furnace and is heated to a temperature
of about 1385.degree. F. to thoroughly melt the metal. The alloy is
treated to remove materials such as dissolved hydrogen and
non-metallic inclusions which would impair casting of the alloy and
the quality of the finished sheet. The alloy can also be filtered
to further remove non-metallic inclusions from the melt.
The melt is then cast through a nozzle and into the casting cavity.
The nozzle is typically fabricated from a refractory material and
provides a passage from the melt to the caster wherein the molten
metal is constrained by a long narrow tip upon exiting the nozzle.
For example, a nozzle tip having a thickness of from about 10 to
about 25 millimeters and a width of from about 254 millimeters to
about 2160 millimeters can be used. The melt exits the tip and is
received in a casting cavity formed by opposite pairs of rotating
chill blocks.
The metal cools as it travels within the casting cavity and
solidifies by transferring heat to the chill blocks until the strip
exits the casting cavity. At the end of the casting cavity, the
chill blocks separate from the cast strip and travel to a cooler
where the chill blocks are cooled. The rate of cooling as the cast
strip passes through the casting cavity of the casting apparatus is
a function of various process and product parameters. These
parameters include the composition of the material being cast, the
strip gauge, the chill block material, the length of the casting
cavity, the casting speed and the efficiency of the block cooling
system.
It is preferred that the cast strip exiting the block caster be as
thin as possible to minimize subsequent working of the strip.
Normally, a limiting factor in obtaining minimum strip thickness is
the thickness and width of the distributor tip of the caster. In
the preferred embodiment of the present invention, the strip is
cast at a thickness of from about 12.5 millimeters to about 25.4
millimeters and more preferably about 19 millimeters.
Upon exiting the caster, the cast strip is then subjected to hot
rolling in a hot mill. A hot mill includes one or more pairs of
oppositely rotating rollers having a gap therebetween that reduce
the thickness of the strip as it passes through the gap. The cast
strip preferably enters the hot mill at a temperature in the range
of from about 850.degree. F. to about 1050.degree. F. According to
the process of the present invention, the hot mill preferably
reduces the thickness of the strip by at least about 70 percent and
more preferably by at least about 80 percent. In a preferred
embodiment, the hot mill includes 2 pairs of hot rollers and the
percentage reduction in the hot mill is maximized. The hot rolled
strip preferably exits the hot mill at a temperature in the range
from about 500.degree. F. to about 750.degree. F. In accordance
with the present invention, it has been found that a relatively
high reduction in gauge can take place in each pass of the hot
rollers and therefore the number of pairs of hot rollers can be
minimized.
The hot rolled strip is optionally annealed to remove any residual
cold work resulting from the hot mill operation and to reduce the
earing. Preferably, the hot rolled strip is annealed in a hot mill
anneal step at a temperature of a minimum of at least about
700.degree. F. and more preferably a minimum of at least about
800.degree. F., and preferably with a maximum temperature of at
most about 900.degree. F. and more preferably a maximum temperature
of at most about 850.degree. F. According to one embodiment, a
preferred temperature for annealing is about 825.degree. F. The
entire metal strip should preferably be at the annealing
temperature for at least about 0.5 hours, more preferably at least
about 1 hour and more preferably at least about 2 hours. The amount
of time that the entire metal strip should be at the annealing
temperature should preferably be a maximum of at most about 5
hours, more preferably a maximum of at most about 4 hours. In a
preferred embodiment, the anneal time is about 3 hours. For
example, the strip can be coiled, placed in an annealing furnace,
and held at the desired anneal temperature for from about 2 to
about 4 hours. This length of time insures that interior portions
of the coiled strip reach the desired annealing temperature and are
held at that temperature for the preferred period of time. It is to
be expressly understood that the annealing times listed above are
the times for which the entire metal strip is maintained at the
annealing temperatures, and these times do not include the heat-up
time to reach the anneal temperature and the cool-down time after
the anneal soak. The coiled strip is preferably cooled
expeditiously to allow further processing, but is not rapidly
quenched to retain a solution heat treated structure.
Alternatively, the hot rolled strip is not subjected to a hot mill
anneal step. In this alternative embodiment, the hot rolled strip
is allowed to cool and is subsequently subjected to cold rolling
without any intermediate thermal treatment. It is to be expressly
understood that the hot rolled strip is not subjected to a heat
soak homogenization, nor is it subjected to a solution heat
treatment followed by a rapid quench. The strip is cooled in the
manner that is most convenient.
After the hot mill annealed or hot rolled sheet has cooled to
ambient temperature, it is cold rolled in a first cold rolling step
to an intermediate gauge. Preferably, cold rolling to intermediate
gauge includes the step of passing the sheet between one or more
pairs of rotating cold rollers (preferably 1 to 3 pairs of cold
rollers) to reduce the thickness of the strip by from about 35
percent to about 60 percent per pass through each pair of rollers,
more preferably by from about 45 percent to about 55 percent per
pass. The total reduction in thickness is preferably from about 45
to about 85 percent. In accordance with the process of the present
invention, it has been found that a relatively large reduction in
the gauge of the aluminum sheet can take place in each pass as
compared to a commercially available continuously cast can stock.
In this manner, it is possible to reduce the number of passes
required in the cold mill.
When the desired intermediate anneal gauge is reached following the
first cold rolling step, the sheet is intermediate cold mill
annealed to reduce the residual cold work and lower the earing.
Preferably, the sheet is intermediate cold mill annealed at a
minimum temperature of at least about 600.degree. F., more
preferably at a minimum temperature of at least about 650.degree.
F., and preferably at a maximum temperature of no more than about
900.degree. F. and more preferably at a maximum temperature of no
more than about 750.degree. F. According to one embodiment, a
preferred annealing temperature is about 705.degree. F. The anneal
time is preferably a minimum of at least about 0.5 hours and is
more preferably a minimum of at least about 2 hours. According to
one embodiment of the present invention, the intermediate cold mill
anneal step can include a continuous anneal, preferably at a
temperature of from about 800.degree. F. to about 1050.degree. F.
and more preferably at a temperature of about 900.degree. F. It has
unexpectedly been found that these cold mill annealing temperatures
lead to advantageous properties.
After the cold rolled and intermediate cold mill annealed sheet has
cooled to ambient temperature, a final cold rolling step is used to
impart the final properties to the sheet. The preferred final cold
work percentage is that point at which a balance between the
ultimate tensile strength and the eating is obtained. This point
can be determined for a particular alloy composition by plotting
the ultimate tensile strength and earing values against the cold
work percentage. Once this preferred cold work percentage is
determined for the final cold rolling step, the gauge of the sheet
during the intermediate annealing stage and, consequently, the cold
work percentage for the first cold roll step can be determined and
the hot mill gauge can be optimized to minimize the number of
passes.
In a preferred embodiment the reduction to final gauge is from
about 45 to about 80 percent, preferably in one or two passes of
from about 25 to about 65 percent per pass, and more preferably a
single pass of 60 percent reduction. When the sheet is fabricated
for drawn and ironed container bodies, the final gauge can be, for
example, from about 0.0096 inches to about 0.015 inches.
An important aspect of the present invention is that the aluminum
sheet product that is produced in accordance with the present
invention can maintain sufficient strength and formability
properties while having a relatively thin gauge. This is important
when the aluminum sheet product is utilized in making drawn and
ironed containers. The trend in the can-making industry is to use
thinner aluminum sheet stock for the production of drawn and ironed
containers, thereby producing a container containing less aluminum
and having a reduced cost. However, to use thinner gauge aluminum
sheet stock the aluminum sheet stock must still have the required
physical characteristics, as described in more detail below.
Surprisingly, a continuous casting process has been discovered
which, when utilized with the alloys of the present invention,
produces an aluminum sheet stock that meets the industry
standards.
The aluminum alloy sheet produced according to the preferred
embodiment of the present invention is useful in a number of
applications including, but not limited to, drawn and ironed
container bodies. When the aluminum alloy sheet is to be fabricated
into drawn and ironed container bodies, the alloy sheet preferably
has an after-bake yield strength of at least about 37 ksi, more
preferably at least about 38 ksi, and more preferably at least
about 40 ksi. After-bake yield strength refers to the yield
strength of the aluminum sheet after being subjected to a
temperature of about 400.degree. F. for about 10 minutes. This
treatment simulates conditions experienced by a container body
during post-formation processing, such as the washing and drying of
containers, and drying of films or paints applied to the container.
Preferably, the as rolled yield strength is at least 38 ksi and
more preferably at least 39 ksi, and preferably is not greater than
about 44 ksi and more preferably is not greater than about 43 ksi.
The aluminum sheet preferably has an after bake ultimate tensile
strength of at least about 40 ksi, more preferably at least about
41.5 ksi and more preferably at least about 43 ksi. The as rolled
ultimate tensile strength is preferably at least 41 ksi and more
preferably at least 42 ksi and more preferably at least 43 ksi, and
preferably, not greater than 46 ksi and more preferably not greater
than 45 ksi and more preferably not greater than 44.5 ksi.
To produce acceptable drawn and ironed container bodies, aluminum
alloy sheet should have a low earing percentage. A typical
measurement for earing is the 45.degree. earing or 45.degree.
rolling texture. Forty-five degrees refers to the position on the
aluminum sheet which is 45.degree. relative to the rolling
direction. The value for the 45.degree. eating is determined by
measuring the height of the ears which stick up in a cup, minus the
height of valleys between the ears. The difference is divided by
the height of the valleys times 100 to convert to a percentage.
Preferably, the aluminum alloy sheet, according to the present
invention, has a tested earing of less than about 2 percent and
more preferably less than about 1.8 percent. Importantly, the
aluminum alloy sheet product produced in accordance with the
present invention should be capable of producing commercially
acceptable drawn and ironed containers. Therefore, when the
aluminum alloy sheet product is converted into container bodies,
the eating should be such that the bodies can be conveyed on the
conveying equipment and the eating should not be so great as to
prevent acceptable handling and trimming of the container
bodies.
In addition, the aluminum sheet should have an elongation of at
least about 2 percent and more preferably at least about 3 percent
and more preferably at least about 4 percent. Further, container
bodies fabricated from the alloy of the present invention having a
minimum dome reversal strength of at least about 88 psi and more
preferably at least about 90 psi at current commercial
thickness.
EXAMPLES
In order to illustrate the advantages of the present invention, a
number of aluminum alloys were formed into sheets.
Four examples comparing AA 3004/3104 alloys with the alloys of the
present invention are illustrated in Table I.
TABLE I
__________________________________________________________________________
Hot mill Cold mill Composition (weight %) Anneal Anneal Secondary
Example Mg Mn Cu Fe Temperature Temperature Cold Work
__________________________________________________________________________
1 (comparative) 1.21 0.84 0.22 0.44 825.degree. F. 705.degree. F.
75% 2 (comparative) 1.28 0.96 0.21 0.41 825.degree. F. 705.degree.
F. 75% 3 1.22 0.83 0.42 0.35 825.degree. F. 705.degree. F. 64% 4
1.31 0.99 0.41 0.34 825.degree. F. 705.degree. F. 61%
__________________________________________________________________________
In each example, the silicon content was between 0.18 and 0.22 and
the balance of the composition was aluminum. Each alloy was
continuously cast in a block caster and was then continuously hot
rolled. The hot mill and intermediate cold mill anneals were each
for about 3 hours. After the hot mill anneal, the sheets were cold
rolled to reduce the thickness by from about 45 to 70 percent in
one or more passes. After this cold rolling, the sheets were
intermediate cold mill annealed at the temperature indicated.
Thereafter, the sheets were cold rolled to reduce the thickness by
the indicated percentage. Table II illustrates the results of
testing the processed sheets.
TABLE II ______________________________________ As-Rolled
After-Bake Elonga- Elonga- Example UTS YS tion Earing UTS YS tion
______________________________________ 1 (comparative) 41.3 39.3
3.2% 2.2% 40.0 35.2 4.8% 2 (comparative) 43.2 40.4 3.1% 2.2% 40.7
36.0 4.3% 3 42.4 39.4 3.2% 1.4% 42.3 37.1 5.1% 4 43.1 40.1 3.2%
1.2% 43.3 37.8 5.3% ______________________________________
The ultimate tensile strength (UTS), yield strength (YS),
elongation, and eating were each measured when the sheet was in the
as-rolled condition. The UTS, YS and elongation were then measured
after a bake treatment which consisted of heating the alloy sheet
to about 400.degree. F. for about 10 minutes.
Comparative Examples 1 and 2 illustrate that, when fabricated using
a continuous caster, an AA 3004/3104 alloy composition is too weak
for can-making applications. In order to achieve similar as-rolled
strengths, the 3004/3104 alloy requires more cold work, and
therefore, has higher earing. Further, the 3004/3104 alloy has a
large drop in yield strength after the bake treatment, which can
result in a low dome reversal strength for the containers.
Examples 3 and 4 illustrate alloy compositions according to the
present invention. The sheets had a significantly lower drop in
yield strength due to baking and therefore maintained adequate
strength for can-making applications. Further, these alloy sheets
maintained low earing. These examples substantiate that AA3004/3104
alloys that are processed in a continuous caster are too weak for
use as containers, particularly for carbonated beverages. However,
when the copper level is increased according to the present
invention, the sheet has sufficient strength for forming cans.
To further illustrate the advantages of the present invention, a
number of examples were prepared to demonstrate the effect of
increased thermal treatment temperature, such as at temperatures
taught by the prior art. These examples are illustrated in Table
III.
TABLE III ______________________________________ Composition Hot
mill Example Mg Mn Cu Fe Anneal Result
______________________________________ 5 1.28 0.98 0.42 0.35
1000.degree. F. Unable to unwrap 3 hours coils 6 1.28 0.98 0.42
0.35 950.degree. F. Unable to unwrap 3 hours coils 7 1.28 0.98 0.42
0.35 925.degree. F. Unable to unwrap 10 hours 4 of 5 coils
______________________________________
As is illustrated in Table III, annealing temperatures at
925.degree. F. or higher resulted in welded coils which were not
able to be unwrapped for further processing. As a result, such
temperatures are clearly not useful for alloy sheets according to
the present invention.
Table IV illustrates the effect of increasing the iron content
according to a preferred embodiment of the present invention.
TABLE IV ______________________________________ Hot mill
Intermediate Composition (weight %) Anneal Cold mill Anneal Example
Mg Mn Cu Fe Temperature Temperature
______________________________________ 8 1.22 0.83 0.42 0.38
825.degree. F. 705.degree. F. 9 1.31 0.94 0.42 0.36 825.degree. F.
705.degree. F. 10 1.37 1.12 0.42 0.55 825.degree. F. 705.degree. F.
______________________________________
In each example in addition to the listed elements, the silicon
content was between 0.18 and 0.23 and the balance was essentially
aluminum. Each alloy was cast in a block caster and was then
continuously hot rolled. The hot mill anneal in all cases was for
about 3 hours. After the hot mill anneal, the sheets were cold
rolled to reduce the thickness by from about 45 to 70 percent in
one or more passes. After this cold rolling, the sheets were
intermediate cold mill annealed for about 3 hours at the
temperatures indicated and then further cold rolled.
Table V illustrates the results of testing the foregoing aluminum
alloy sheets.
TABLE V ______________________________________ UTS YS Elongation
Earing Example (ksi) (ksi) % % Result
______________________________________ 8 42.3 37.0 5.0 1.5
Excellent for 5.5 oz. cans 9 43.2 38.2 4.8 1.6 Made 12 oz. cans 10
43.2 37.8 5.2 1.7 Excellent for 12 oz. cans
______________________________________
The ultimate tensile strength (UTS), yield strength (YS) and
elongation were measured after a bake treatment which consisted of
heating the alloy to about 400.degree. F. for about 10 minutes.
Example 8 illustrates an alloy and process according to the present
invention for making a sheet product which is sufficient for 5.5
ounce can bodies. By increasing the copper content and maintaining
an adequate cold mill anneal temperature, sheet is produced that is
excellent for the commercial production of 5.5 ounce container
bodies. However, the sheet did not have sufficient formability for
the commercial production of 12 ounce container bodies. Although
the sheet had sufficient strength and 12 ounce container bodies
were made, a commercially unacceptable number of the 12 ounce
container bodies were rejected when produced on two commercial
can-lines.
Example 9 is similar to Example 8, with increased magnesium and
manganese; the sheet was also useful for 5.5 ounce container bodies
and did produce some 12 ounce container bodies with acceptable
strength. However, the 12 ounce container bodies also had a
commercially unacceptable number of rejects.
Example 10 illustrates that by increasing the iron content
according to the present invention, this problem can be overcome.
In Example 10, the sheet material had excellent fine grain size and
was used to produce 12 ounce container bodies on two commercial
container lines with a commercially acceptable rate of
rejection.
In an alternative embodiment of the present invention, fine grain
size may be imparted to the sheet material by using a continuous
intermediate cold mill anneal. In one example, an aluminum alloy
sheet having the composition illustrated for Example 4 was
intermediate cold mill annealed in a continuous, gas-fired furnace
wherein the metal was exposed to a peak temperature of about
900.degree. F. This treatment imparted a very fine grain size to
the sheet. The sheet had an ultimate tensile strength of 45.5 ksi
and 12 ounce container bodies were produced that met commercial
strength requirements.
While various embodiments of the present invention have been
described in detail, it is apparent that modifications and
adaptations of those embodiments will occur to those skilled in the
art. It is to be expressly understood that such modifications and
adaptations are within the spirit and scope of the present
invention.
* * * * *